FIG. 1 schematically shows a progressive ophthalmic lens 8 in projection in the mean plane of the frame 10. The expression “progressive lens” (or “progressive eyeglass”) is understood to mean an ophthalmic lens with a progressive cylindrical and/or spherical power addition for visual-correction spectacles. The progressive lens includes, in its upper portion, a far-vision zone 11 the optical power of which is suitable for the far vision of the wearer depending on his visual-correction needs and, in its lower portion, a near-vision zone 12 the optical power of which is suitable for the near vision of this wearer.
As is known, between the far-vision (or FV) zone 11 and the near-vision (or NV) zone 12 there is a vision zone suitable for intermediate-distance vision (or IV, not illustrated).
The reference point IVL is a far-vision point defined by the manufacturer, for example at the center of a circle delimiting the far-vision zone 11. Likewise, the reference point IVP is a near-vision point defined by the manufacturer, for example at the center of a circle delimiting the near-vision zone 12.
Standard ISO 13666:2012 defines certain parameters used for the fitting of a progressive lens. Thus, the fitting point CM is a point located on the front surface of an eyeglass or a semi-finished eyeglass, that the manufacture considers to be a reference point for the positioning of the eyeglass in front of the eye. The fitting point CM is in general marked by an erasable marking that is removed after fitting. The fitting height FH is the vertical distance that separates the fitting point CM from the horizontal tangent passing through the bottom point of the periphery of the edged lens, i.e. through the lowest point of the interior outline of the rim of the frame. The datum height (Hd) is the vertical distance between the fitting point CM and that point of the frame 10 which is located vertically under the fitting point CM. The datum height Hd is smaller than or equal to the fitting height FH, depending on the shape of the edged lens (which corresponds, when the frame is a full-rimmed frame, to the shape of the interior outline of the corresponding rim of the frame 10).
A progression length (LP or LOC for length of corridor) is also defined as the vertical distance between the fitting cross and the position of the point IVP.
Moreover, the internal offset (or inset E) of the lens is defined as the horizontal offset between the far-vision reference point IVL and the near-vision reference point IVP.
The optical power of the lens varies, preferably continuously, between said far-vision reference point IVL and said near-vision reference point IVP, along a (jagged or curved) line called the principal progression meridian line, which passes through these two points. This principal progression meridian line passes through the three FV, IV and NV zones in an overall vertical direction.
The conception of a progressive lens especially aims to determine the position of the points IVL and IVP, the position and extent of the far-vision zone 11, the position and extent of the near-vision zone 12, the value of the internal offset E and the progression profile of the optical power addition along the principal progression meridian line.
The choice of a frame places certain constraints for example on the determination of the progression length Lp.
The optician in general determines values of progression length and of position of the FV and NV zones during optical spectacle-fitting measurements, for example acquired on an apparatus of the type sold under the trademark Visioffice.
Customarily, the choice of the progression length is made by the optician on the basis of subjective criteria such as the posture of the wearer or feedback given by the latter on his preceding piece of equipment.
A method for determining the progression length of a lens is also known from document U.S. Pat. No. 8,297,752, in which method a single far-vision point of the wearer and a single near-vision point of the wearer are determined on the ophthalmic lens and a corresponding progression length is deduced therefrom. An ophthalmic lens suitable for the wearer may thus be selected.
However, depending on the shape of the rims of the frame, it is not certain, by applying this method, that all the zone for near vision used by the wearer will be included in the ophthalmic lens once the latter has been edged and fitted in the frame chosen by the wearer.